Rapid restoration of blood flow is vital to restoring tissue function in many ischemic conditions, such as critical limb ischemia. A variety of strategies have been explored to solve the challenging problem of tissue vascularization, including delivery of one or more pro-angiogenic factors, the use of cell-based therapies, and the transplantation of pre-vascularized tissues. By combining some of the attractive elements of all three of these major strategies with recent advances in modular tissue engineering, this multi-PI project will create an innovative, minimally invasive approach to stimulate revascularization of ischemic tissue in vivo. Endothelial cells (EC) and mesechymal stem cells (MSC) will be encapsulated in biomaterial-based modules to allow formation of stable, pericyte-invested vascular units within these microtissues over time in culture. Inosculation of vessels in adjacent microtissues will be assessed and a candidate mechanism by which neighboring sprouts form anastomoses will be tested. Underlying this mechanism is the hypothesis that inosculation depends in part on cell-generated and matrix- propagated tractional forces between neighboring vascular sprouts. Our overall strategy is to perform vascularized modules in vitro that rapidly inosculate with each other and with host vessels, and thereby restore blood perfusion to an ischemic tissue following delivery in vivo. This strategy will be evaluated by completing three Specific Aims. In Aim 1, we will characterize and quantify the ability of EC and MSC encapsulated within modular biomaterials to develop into primitive vessel-like networks. Aim 2 will focus on the interactions of these prevascularized modules, assessing their ability to inosculate in a model tissue in vitro, and will test a candidate mechanism explaining how nascent capillaries interconnect. In Aim 3, we will compare the therapeutic efficacy of our approach head-to-head with two other cell-based strategies in an established model of hind limb ischemia. Successful completion of these three aims may lead to a new and powerful technique to rapidly restore vascular beds in virtually any ischemic tissue, and thereby offers the potential to broadly impact current clinical approaches to treating peripheral arterial disease, coronary artery disease, and complications due to diabetes. These studies will also improve current understanding of the synergistic roles of EC, MSC, and their microenvironment on capillary morphogenesis, which in turn may lead to important new discoveries that can enhance tissue vascularization strategies.